FR2875138A1 - CONTROL METHOD FOR A HEATING HUMIDIFIER - Google Patents

Abstract

The invention relates to a method for feeding a mask (7) with controlled humidification rate air, said method comprising the steps of: - providing a water reservoir (4) arranged so that the air circulates in contact with the surface of the water and is charged with moisture - providing heating means (3) by circulation of an electric current adapted to heat the water of the tank, the method further comprising the steps of measuring the average intensity of the current flowing through the heating means and regulating the said average intensity around a set point, so as to obtain an air humidification rate that is independent of the ambient temperature.The invention also relates to a device for regulating the humidification rate of the air flow, as well as a heating humidifier comprising such a regulating device.

Description

The invention relates to a method of feeding a mask in air at a rate

  controlled humidification device, a device for regulating the humidification rate of an air flow, and a heated humidifier comprising such a regulating device.

  It is known to place a humidifier at the outlet of a respiratory assistance device delivering air to a user, so as to humidify the air supplied to the user, and thus prevent drying out of the airways.

  In order to obtain sufficient humidification of the air over the entire range of possible air flow (between 0 and 21 / s), it is planned to heat the water contained in a tank of the humidifier in order to accelerate the evaporation and therefore the transfer of the water molecules to the air delivered to the user.

  For the comfort of the user, the humidifier must be able to adjust the humidification rate of the supplied air.

  Control systems for such heated humidifiers are already known, aimed at providing the user with air whose humidity content is in conformity with that desired, for example substantially constant.

  A first known system consists in regulating only the temperature of the water. However, the rate of humidification of the air delivered to the user also depends on the air temperature, and it can vary, especially between day and night. It follows that the simple regulation of the water temperature does not allow to have a constant rate of humidification of the air, which is not satisfactory.

  A second known system uses, on the one hand, a temperature probe for water and, on the other hand, a temperature probe for air. This system is more efficient since it takes into account the ambient conditions. However, this system requires the use of two probes, it is relatively expensive and more complex to implement.

  The object of the invention is to solve these problems.

  For this purpose, and according to a first aspect, the invention relates to a method for supplying an air mask with a controlled humidification rate, said method comprising the steps of: providing a water reservoir arranged so that the air circulates in contact with the surface of the water and is charged with moisture; provide heating means by circulation of an electric current able to heat the water of the tank; the method further comprising the steps of measuring the average intensity of the current flowing through the heating means and controlling said average intensity around a set point, so as to obtain an independent air humidification rate the ambient temperature.

  The heating means can be supplied with a rectified sinusoidal current comprising alternating current and alternations blocked at 0, the regulation of the average intensity of the current flowing through the heating means being obtained by setting the number of alternating pulses during a given time interval.

  According to a possible embodiment, the method comprises the steps of: storing a power setpoint supplied to the heating means, chosen by the user; measuring and storing the peak value of the supply voltage of the heating means; - calculating, from the power setpoint and the peak value of the supply voltage, the set value of the average intensity of the current flowing through the heating means, and storing said set value of the average intensity ; measuring and storing the average intensity of the current flowing through the heating means; compare the measured value of the average intensity with the set value of the average intensity; set the number of passing cycles during a given time interval so as to decrease the difference between the measured value of the average intensity and the set value of the average intensity.

  According to a second aspect, the invention relates to a device for regulating the rate of humidification of a flow of air circulating in contact with the surface of the water contained in a reservoir, and intended to be distributed to a user via a mask, said water being heated by heating means by circulation of an electric current, the device comprising means for measuring the average intensity of the current flowing through the heating means and means for regulating said average intensity around the a set value.

  The device may comprise: means for selection by the user of the desired power, called the desired power; means for measuring the peak value of the supply voltage of the heating means and the average intensity of the current flowing through the heating means; means for storing the desired power and the peak value of the supply voltage; means for calculating the set value of the average intensity of the current flowing through the heating means, from the power setpoint and the peak value of the supply voltage; means for storing the set value of the average intensity and the measured average intensity; comparison means between the measured value and the set value of the average intensity; - Means controlled by the comparison means and able to act on the supply of the heating means so as to reduce the difference between the measured value and the set value of the average intensity.

  According to one possible embodiment, the device comprises a device for rectifying the voltage delivered by the mains and a blocking device capable of blocking at 0 some of the alternations of the voltage, said rectifying and blocking devices being arranged so that the means of heating can be supplied by a rectified sinusoidal current comprising alternating pass and alternations blocked at 0. The blocking device can be arranged to set the number of halfwaves blocked at 0 during a given period of time as a function of the difference between the measured value and the set value of the average intensity.

  Finally, according to a third aspect, the invention relates to a heating humidifier, comprising a water tank, heating means by circulation of an electric current able to heat the water of the tank, an inlet and an outlet of air arranged so that a flow of air can flow in contact with the surface of the water and take up moisture, and a control device as previously described.

  The other features of the invention result from the description which follows of an embodiment, description made with reference to the appended figures in which: FIG. 1 is a perspective view of a heating humidifier according to the invention, comprising a outlet duct associated with a mask applied to the nose and mouth of a user; FIG. 2 is a graphical representation showing the calculated evaporation curve of the water (in g / s) in contact with the air, for an ambient temperature of 25 ° C., as a function of the temperature difference between the water and air, as well as the linear approximation of this curve; FIG. 3 is a graphical representation showing the calculated evolution of the evaporation of water (in g / s) in contact with the air, for an ambient temperature of 25 ° C., as a function of the relative humidity of the air, for different values of temperature differences between water and air; FIG. 4 shows the signal of the current flowing through the heating element of the heating humidifier of FIG. 1 as a function of time; FIG. 5 is a diagram illustrating the operation of the heating humidifier of FIG. 1.

  FIG. 1 shows a heated humidifier 1, interposed between a respiratory assistance device (not shown) and a user 2. For example, the respiratory assistance device may be a medical device for treating the symptoms of the patient. sleep apnea, which can be used in the laboratory or at home.

  The heated humidifier 1 comprises a heating element, such as a metal plate 3 in contact with a resistor.

  The resistor may consist of a screen printed track on an insulated metal support or a flexible film resistor bonded to a metal support. The heating element is designed to provide electrical insulation of 4000Veff between its conductive part (resistance) and its upper face which is an accessible part. The layout of this track (coil) is such that the heat transfer is distributed homogeneously over the entire metal surface in contact with the water tank of the humidifier 1.

  A water tank 4 is placed on the heated metal plate, a spring system (not shown) holding the plate in contact with the bottom of the tank 4 and thereby improving the heat transfer.

  The tank 4 is equipped with an inlet, through which air enters from the respiratory assistance apparatus, and an outlet 5 connected to a duct 6 at the end of which is associated a mask 7 intended to be applied on the nose and / or mouth of the user 2.

  The various components of the heating humidifier 1 are housed in a housing 8 of insulating material, for example plastic, provided with selection members and control 9 can be actuated by the user.

  The air delivered by the respiratory assistance device passes through the heating humidifier 1 so as to be charged with moisture in contact with the surface of the water contained in the reservoir 4. The humidified air leaving the The heated humidifier 1 is then directed to the user 2 via the duct 6 and the mask 7 (see the arrows shown in FIG. 1).

  The heated humidifier is designed to operate normally under the following conditions: atmospheric pressure: 700 hPa to 1060 hPa temperature: +5 C to +35 C relative humidity: 15% to 95% without condensation The operating temperature is limited to 35 C to meet the requirement of the standard which imposes a maximum temperature of the air delivered to the user of 41 C.

  In order to obtain a desired humidification of the air supplied, the invention provides for controlling the power delivered to the heating element.

  It will be shown below that the power regulation makes it possible to obtain a constant humidification rate, whatever the ambient temperature, and without the necessity of using a temperature probe.

  If we consider a water surface in contact with a mass of air, the transfer of water to the air, under the conditions of use of the humidifier, can be described by the Incropera equation. and Dewitt: m = (S / Rw) (h / Cp Le ('-) ((Ps (Ta) / Ts) (Pv (Ta) / Ta)) Equation 1 where: m = mass of water transferred per unit time (in g / s) S = water / air exchange surface (m) Rw = 462 j / kg.K (water-related constant) h = heat transfer coefficient from water to water air (depends on the system) Cp = 1008 j / kg.K (specific heat of water) = 0.846 (Lewis constant) n = 3 (empirically determined coefficient in the case of water) Ps (Ts) = 611 exp (17.27 x Ts / (237.3 + Ts)) (saturating pressure of water vapor at temperature Ts) Ts = surface water temperature (in C) Pv (Ta ) = Ps (Ta) X HR (water vapor pressure at ambient temperature and humidity) Ta = air temperature (in C) RH = relative humidity of the air (between 0 and 1) The factors involved in the transfer of mass and their influence on it are as follows: The surface S of water / air exchange: Any other condition remaining constant, and in particular the water and air temperatures. air, equation 1 is equivalent to: m = cte x S. The humidification rate is therefore a linear function of the water / air exchange surface.

  The difference in temperature between water and air (Ts Ta): If water and air are at the same temperature, there is a transfer of water molecules to the air (natural convection) that depends only the moisture content of the air. This transfer is not zero if the relative humidity of the air is less than 100%.

  If the water is heated to raise its temperature a few degrees, there is a larger transfer, the humidification rate (mass of water transferred m) being proportional to the temperature difference between the water and air, as shown in Figure 2.

  The curve of FIG. 2 was obtained by calculation with equation 1, for the following values (parameters close to those of the application envisaged): 5 = 0.01 m2 h = 20 p = 1.2 g / 1 (density of air) Ta = 25 C It can be seen that, for the user, the sensation of humidity of the air can be considered as a linear function of the temperature difference between water and air (Linear approximation of the curve of Figure 2 with a correlation coefficient R = 0.952).

  io - Humidity of the ambient air (HR): The curves of FIG. 3 were obtained by calculation with equation 1, for the following values: S = 0.01 m2 h = 10 Ta = 25 C Ts = 25, 35, 50, 65 C HR ranging from 10 to 90% It is found that the humidification rate m is a linear function of the relative humidity of the ambient air.

  The air flow rate (V ', in I / s, considered constant and uniform): Equation 1 describes the exchange of water molecules in a static manner. However, in the humidifier, the tank is traversed by the air, so that during the flow, the ambient air replaces the air that has just been moistened, thereby increasing the humidification rate. compared to the static case.

  According to the first law of thermodynamics, we have: P = (Ts Ta) / Rth Equation 2 where P is the power supplied to the system and Rth the thermal resistance of the system expressed in K / W.

  The thermal resistance Rth only depends on the dimensions of the heat exchange system (its thickness e and the exchange surface S) and heat transfer coefficients depending for example on the nature of the materials in the presence. This thermal resistance is therefore a fixed characteristic of the system. Since the thermal resistance is inversely proportional to the exchange surface, we can write: Rth = cte / S. As a result, equation 2 can be written as: P / [(Ts Ta) x S] = cte Equation 3 Thus, the regulation of the power makes it possible to keep a product (Ts Ta) x S constant.

  That is, for a given exchange surface, the temperature difference between water and air is a linear function of the power supplied to the system. However, as indicated above, the humidification rate depends substantially linearly on this difference in temperature. As any other parameter remains constant, it is therefore possible to write that: m cte x P. On the other hand, for a constant power, the possible variation of the exchange surface, due for example to the variable section of the reservoir as a function of the quantity of liquid it contains, is automatically compensated by an inversely proportional variation of the factor (Ts Ta) which makes it possible to leave unchanged the constant above binding m and P. Consequently, the regulation of the power makes it possible to guarantee a rate humidification independent of the ambient temperature Ta and the exchange surface of the tank.

  This humidification rate nevertheless remains dependent on the ambient humidity RH and the flow rate of air passing through the apparatus (the heat transfer characteristics of the heating element and the reservoir being fixed for a given system).

  We now describe how the invention provides for regulating the power supplied to the system.

  The general principle is to regulate the power by passing more or less alternations of a rectified sinusoidal signal obtained from the mains voltage. This has the advantage of allowing adaptation to all possible mains voltages by compensating a low voltage by a greater number of alternating pass and vice versa.

  The signal delivered to the heating element is shown in FIG. 4. It is a sinusoidal rectified double amplitude signal of amplitude A and of period T, canceled between the instants XT and ZT. We can therefore write: between O and XT: I (t) = A sin (ce) with co = bed / 2T = t / T between XT and ZT: 1 (t) = O. We denote by alternation the half period of the sinusoidal signal, ie the period of the sinusoidal signal being rectified.

  X is the number of passing alternations and Z is the total number of alternations 15 (busy and blocked).

  The electric power delivered to the heating element is: P = Ueff x Ieff = R x leff2 Equation 4 Gold, Ieff = zT xf f2 (t) dt and Imoy = ZT xff (t) dt 0 0 So: Ieff 2 = ZT xf A2 - (1 cos (2wt)) dt 2 T Ieff 2 = A x X f (1 cos (2e xt)) dt 2T Z0 T A2 X leff2 = x 2 Z Imoy 2A X 2Z 4A 4 = xx = _ Ieff 2 r Z A2X A27c Ayr And: Ieff 2 = A_ x Im oy

XT

Imoy ZT x Asin (wt) dt o

T

  Imoy = A x X f sin (- x t) dt T Z0 T Imoy = 2AxX

Z From where

  Equation 4 can be written as: P _ RxImoyxImaxxJr _ UmaxxImoyxir because R x Imax = Umax 4 4 Since the peak value of the mains voltage (Umax) is constant, we have: P = cte x Imoy The principle of regulation is therefore the following: the desired power Pc is set by the user of the heating element, according to his preferences. The set value of Imoy (Imoyc) is then determined by the system, knowing Umax. The values of Pc, Imoyc and Umax are digitized by a microcontroller.

  The actual average current flowing through the heating element, Imoy, is measured using a simple measuring resistor connected in series with the heating element. The microcontroller then compares Imoy with Imoyc and determines whether the number of alternations crossing the heating element must be increased or decreased so that the value of Imoy is stable and equal to Imoyc. For example, if Imoy is greater than Imoyc, it is necessary to decrease the number of alternations crossing the heating element.

  Referring now to Figure 5 which shows schematically the heating humidifier, as well as the supply means and the associated control means.

  The humidifier 1 comprises an electronic card intended to be powered by the mains voltage 10 via an IEC type socket C8. A specific cord supplied with the heated humidifier allows it to be connected to all the available networks (varying according to the country).

  At the input of the electronic card is provided a device 11 for filtering / protecting and rectifying the mains voltage 10.

  Thus, a bipolar switch 11a is provided successively, making it possible to cut off the electrical supply on the electronic card, two fuses 11b of 2A protecting the electronic card, a filter 11c constituted by a capacitance, to avoid problems of electromagnetic compatibility, and a varistor 11d mounted in parallel on the mains input to shunt any overvoltages carried by the sector.

  The mains voltage 10 undergoes a full-wave rectification, so that the resistance of the heating element is supplied by a current as shown in FIG. 4. The rectified voltage is also used to create a supply 12 in two direct voltages V 1 and V2. Thus, a ballast transistor 12a, a voltage reference 12b and a regulator 12c are provided.

  The voltage V2 is used for the control 16 of a transistor 17 for controlling the heating element and for supplying an operational amplifier (see below), while the voltage V1 supplies a microcontroller 13.

  The regulation of the power delivered to the heating element is carried out by the microcontroller 13 embedded in the heating humidifier 1.

  The microcontroller 13 is in a housing. It has a ROM, Flash, RAM and EEPROM. It contains a built-in 10-bit analog / digital converter with 4 multiplexed analog channels, an integrated analog comparator and an internal 4MHz oscillator. It also contains a voltage drop detector and a watchdog.

  The power setpoint Pc is set by a potentiometer 14 accessible from outside the housing of the humidifier 1 so that the user 2 can choose the desired power, and therefore the desired humidification rate. The potentiometer 14 is graduated from 1 to 5: the position 5 (maximum stop slider) delivers the maximum heating power and the position 1 (slider at minimum stop) a power equal to 20% of the maximum power, the other positions delivering a power proportional to the graduation. The use of the humidifier 1 without heating requires, if necessary, to cut the power supply of the humidifier using a switch.

  The potentiometer 14 is powered by V1 which ensures that the associated analog input never sees a voltage greater than V1. Its value is chosen to be compatible with the input impedance of the microcontroller 13. The signal of the potentiometer 14, freed from its possible parasites by a filtering capacitor 21, is directly digitized by the microcontroller 13.

  The supply voltage (Umax) of the heating element is also measured by the microcontroller 13, which digitizes an analog voltage taken from the midpoint of a resistor bridge powered by the voltage from the full wave rectification. The resistor bridge has been calculated so that the voltage applied to the analog input of the microcontroller 13 is always lower than V1 regardless of the mains voltage used to supply the humidifier 1.

  The current flowing through the heating element (Imoy) is measured by measuring the voltage across a small power resistance. The signal obtained on this resistance is first filtered by a capacitor 18 to remove the noise and then amplified by an operational amplifier 19 whose gain is calculated so that the signal remains below V1 regardless of the operating mode of the assembly.

  The output of the amplifier then passes through a low-cut filter (resistance / capacitance) with a low cut-off frequency to recover an analog signal image of the average value of the current (Imoy) flowing through the heating element. This signal is finally digitized by the microcontroller 13.

  Finally, the temperature T of the heated plate 3 is measured by means of a CTN type thermistor placed in the center of the lower face of the plate 3. This thermistor is included in a linearization bridge consisting of two resistors and fed by V1. The voltage from this bridge is then digitized by the microcontroller 13.

  The heating element 3 is controlled by an enriched N-channel MOSFET transistor 17. It allows the current to pass through the heating resistor when a voltage VGS is applied to its gate.

  The advantage of such a component is to have an extremely low on-state resistance (RDSon) which avoids a loss of power at the level of the heating element and limits the temperature rise of the transistor.

  In order to obtain the best possible RDS resistance, the VGS voltage must not be less than 10V. But the digital control signal from the microcontroller is limited.

  An operational amplifier, powered by V2, is used as a comparator, with a nominal switching threshold of 2.96V, to transform the signal of the microcontroller into a maximum value signal V2 which is applied to the gate of the MOSFET. The low impedance of the output of the amplifier also makes it possible to obtain fast switching times of the transistor.

  A calibrated safety thermostat 15 is connected in series with the heating resistor. This thermostat 15 which is in direct contact with the metal plate 3 makes it possible to interrupt the heating if the temperature T exceeds the safety value. When the thermostat 15 is triggered, the electronic card continues to operate normally but the resistance is no longer supplied. Only a mechanical action can reset the thermostat.

  Apart from the heating element, all electronic components are implanted directly on the printed circuit board. The heating element is connected to the electronic board by connectors to simplify the assembly and reduce the assembly time. The card is intended to be installed simply in the lower half-shell of the humidifier 1 and held by means of plastic clips.

  The regulation program is now described, the purpose of which is to deliver a heating power P equal to a set power Pc, adjustable by the user 2, whatever the mains voltage 10 (85 to 264 Vac). The passage or blocking alternations is performed by an electronic control member capable of cutting off the supply of the heating element, as indicated above.

  The values of Pc, Umax, Imoy and temperature T of the plate 3 are digitized by the microcontroller 13, via the analog / digital converter, these values being then exploited by the onboard program.

  Initially, the microcontroller 13 passes a given number of alternations out of 100 through the heating element. The program proceeds as follows.

  In order to be synchronized with the cycles of the mains wave, the microcontroller 13 detects the zero crossing of the rectified wave. Thus the mains voltage will not be brutally chopped.

  The microcontroller 13 measures the value Umax of the mains voltage while waiting for the transition to zero, and adds this value to that previously measured. Once it has detected the zero crossing, the microcontroller 13 decrements the number of passing half-cycles (X) and the total number of half-waves counter (Z). If the number of passing alternations is equal to 0, then the microcontroller 13 blocks the alternations. In the opposite case, the microcontroller 13 passes the alternations.

  The microcontroller 13 then measures: the value of the current flowing through the heating element (Imoy), and adds this value to that previously measured; as well as the temperature T of the plate 3, and adds this value to that previously measured.

  The microcontroller 13 then waits for the mains wave to be output from its zero crossing to restart the cycle.

  When the total number of alternations is equal to zero, the microcontroller 13 calculates the average current which passes through the heating element (Imoy), calculates the value of the mains voltage Umax, acquires the setpoint value (Pc) , calculates the value of the safety current, compares the value of the measured current (Imoy) with that of setpoint (Imoyc) and safety and calculates the value of the average temperature.

  If Imoy <Imoye, Imoy should be increased and, for this purpose, the microcontroller will let one more alternation through the heating element. On the other hand, if Imoy <Imoyc, the microcontroller will let one less alternation pass through the heating element. In case of exceeding the safety current or a temperature T

  greater than, for example, 70 C, the microcontroller blocks the passage of alternations. On the other hand, if the current exceeds the maximum allowable current, the microcontroller goes into a waiting routine and will do nothing but refresh the watchdog. If the temperature exceeds 70 C, the microcontroller gets into a routine where it will constantly measure the temperature and refresh the watchdog. If the microcontroller measures a temperature below 65 C, it goes back into the main loop.

  In summary, the treatment phase includes: the decrementation of the counters (all the alternations) - the measurement of the current (all the alternations) the measurement of the plate temperature (all the alternations) - the calculation of the average current (every 100 alternations) the calculation of the peak voltage (every 100 alternations) the calculation of the safety current (every 100 alternations) the calculation of the average temperature (every 100 alternations) the measurement of the setpoint Pc (every 100 alternations) ) the updating of the variables (every 100 alternations) the comparison of the average current with the setpoint current and the safety current (every 100 alternations).

Claims (8)

  A method of feeding a mask (7) with controlled humidification rate (m) air, said method comprising the steps of: providing a water reservoir (4) arranged to circulate the air contact with the surface of the water and take charge of moisture; providing heating means (3) by circulation of an electric current adapted to heat the water of the tank; characterized in that it further comprises the steps of measuring the average intensity (Imoy) of the current flowing through the heating means and regulating said average intensity (Imoy) around a set value (Imoy), so to obtain a humidification rate (m) of the air independent of the ambient temperature (Ta).   2. Method according to claim 1, characterized in that supplies the heating means (3) with a rectified sinusoidal current comprising alternating pass and alternations blocked at 0, the regulation of the average intensity (Imoy) of the current passing through the heating means being obtained by setting the number of passing alternations during a given time interval.   3. Method according to claim 2, characterized in that it comprises the steps of: storing a power setpoint (Pc) supplied to the heating means (3), chosen by a user (2); measuring and storing the peak value (Umax) of the supply voltage of the heating means; calculating, from the power setpoint (Pc) and the peak value (Umax) of the supply voltage, the set value (Imoy) of the average intensity of the current flowing through the heating means, and storing said set value (Imoy) of the average intensity; measuring and storing the average intensity (Imoy) of the current flowing through the heating means; compare the measured value of the average intensity (Imoy) and the set value (Imoyc) of the average intensity; setting the number of passing cycles during a given time interval so as to decrease the difference between the measured value (Imoy) of the average intensity and the set value (Imoyc) of the average intensity.   4. Device for regulating the humidification rate (m) of a flow of air flowing in contact with the surface of the water contained in a tank (4), and intended to be distributed to a user (2) via a mask (7), said water being heated by heating means (3) by circulation of an electric current, characterized in that it comprises means for measuring the average intensity (Imoy) of the current flowing through the heating means and means for regulating said average intensity (Imoy) around a set value (Imoyc).   5. Device according to claim 4, characterized in that it comprises: means (14) for selecting by the user (2) the desired power, called the desired power (Pc); means for measuring the peak value (Umax) of the supply voltage of the heating means (3) and the average intensity (Imoy) of the current flowing through the heating means; means for storing the nominal power (Pc) and the peak value (Umax) of the supply voltage; means for calculating the set value (Imoyc) of the average intensity of the current flowing through the heating means, from the power setpoint (Pc) and the peak value (Umax) of the supply voltage; means for memorizing the set value (Imoy) of the average intensity and the measured average intensity (Imoy); comparison means between the measured value (Imoy) and the set value (Imoyc) of the average intensity; means controlled by the comparison means and able to act on the supply of the heating means (3) so as to reduce the difference between the measured value (Imoy) and the set value (Imoyc) of the average intensity .   6. Device according to claim 4 or 5, characterized in that it comprises a rectifying device (11) of the voltage (10) delivered by the sector and a blocking device able to block at 0 some of the alternations of the voltage said rectifying and blocking devices being arranged so that the heating means (3) can be powered by a rectified sinusoidal current comprising alternating current and half-cycles blocked at 0.   7. Device according to claim 6, characterized in that the blocking device is arranged to set the number of halfwaves blocked at 0 during a given time interval as a function of the difference between the measured value (Imoy) and the value setpoint (Imoyc) of the average intensity.   8. Heated humidifier, comprising a water tank (4), heating means (3) by circulation of an electric current adapted to heat the water of the tank, an inlet and an outlet (5) arranged air so that a flow of air can circulate in contact with the surface of the water and be charged with moisture, characterized in that it further comprises a device for regulating the rate of humidification of the air flow according to one of claims 4 to 7.